Previously, chelates have been attached to peptides and proteins by means of amide bonds derived from the use of active esters and anhydrides (e.g. DTPA-anhydride). The problem with this approach is that the reaction conditions must be partially non-aqueous to avoid hydrolysis of the active ester or anhydride and many peptides/proteins are damaged by these conditions. In this invention, the problem is solved by means of chelates containing isothiocyanate groups which have higher affinity and which allow simpler coupling to peptides and proteins, e.g., at pH 8.5 in aqueous solutions. The coupling chemistry is simple and stoichiometric. The chelates are based on the hydroxybenzylethylenediamine-diacetic acid (HBED), hydroxybenzylpropylenediamine-diacetic acid (HBPD) and hydroxybenzyl-ethylenetriamine-diacetic acid (HTDD) nucleus which offer greater affinity for .sup.111 In and .sup.67 Ga diethylene-triamine pentacetic acid (DTPA) chelates.